1. Field of the Invention
The present invention relates to a scanning device for projecting a light beam against a target having information contained thereon which is to be scanned by the light beam; and in particular relates to a compact laser scanning device, such as a bar code reader projecting a laser beam, and which device incorporates a drive component such as a scanning motor for oscillating a scanning element or mirror at a high rate of speed within a specified angular range so as to cause the light or laser beam which impinges against the information on the target to be rapidly swept across the information in a scanning sequence. In particular, the invention is directed to improving the linearity of the oscillatory or "dithering" motion imparted by the scanning motor to the scanning element or mirror. The invention also contemplates a novel method of utilizing the "dithering" or oscillating motion of the scanning motor for effectuating an improvement in the quality of the scan by the scanning device.
The utilization of laser scanning devices for the scanning or reading of information provided on a target; such as a package or sale item, is well known in this particular technology and has found wide acceptance in commerce. In this connection, various types of laser scanning devices incorporate scanning heads which house optical reading systems, such as bar code readers, for the reading of information or bar code symbols on targets which are scanned by a laser beam projected from the bar code reader. In general, such laser scanning devices; especially those in the type of bar code readers, are widely employed in industry, such as manufacturing, shipping, and in retail commerce and; for example, may be permanently incorporated in the structures of check-out counters of supermarkets, whereby the items of merchandise having the bar code symbols imprinted thereon or applied thereto are passed over a fixed bar code reader located beneath the counter surface so as to provide a record for the merchant of the merchandise being purchased by a consumer, and concurrently a readout (and possibly a printed record) for the consumer.
Alternatively, the bar code reader or laser scanning device may also be constituted of an optical scanner unit which is fixedly mounted on a stand extending above a support platform or countertop on which the merchandise may be arranged; or in many instances of utilization, pursuant to a preferred embodiment of the invention, may be in the form of a miniature, lightweight and gun-shaped device having a pistol grip, and which the activated device is normally passed over the bar code symbol which is imprinted on a sale item or target at some short distance therefrom so as to enable scanning of the information provided by the bar code symbols.
2. Discussion of the Prior Art
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprises of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light-reflecting characteristics. The readers and scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form and utilized as an input to a data processing system for applications, in point-of-sale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; and 4,896,026, all of which have been assigned to the same assignee.
As disclosed in some of the above patents, one embodiment of such a scanning system resides, inter alia, in a hand-held, portable laser scanning head supported by a user, which is configured to allow the user to aim the head, and more particularly, the light beam or laser beam projected therefrom, at a target and a symbol which is to be read.
The light source in a laser scanner is typically a gas laser or semiconductor laser. The use of semiconductor devices, such as a laser diode, as the light source in scanning systems is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a lens, to form a beam spot of a certain size at the target distance. It is preferred that the beam spot size at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
Bar code symbols are formed from bars or elements that are typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies a unique "start" and "stop" character is used to indicate where the bar code begins and ends. A number of different bar code symbologies exist. These symbologies include UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5.
For purpose of discussion, characters recognized and defined by a symbology shall be referred to as legitimate characters, while characters not recognized and defined by that symbology are referred to as illegitimate characters. Thus, an arrangement of elements not decodable by a given symbology corresponds to an illegitimate character(s) for that symbology.
In the scanning systems known in the art, the light beam is directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The scanning functions by repetitively scanning the light beam in a line or series of lines across the symbol. The scanning component may incorporate a drive or scanning motor adopted to either sweep the beam spot across the symbol and trace a scan line across and past the symbol in a high-speed repetitive mode, or scan the field of view of the scanner, or do both.
Scanning systems also normally include a sensor or photodetector which functions to detect light reflected from the symbol.
The photodetector is therefore positioned in the scanner or in an optical path in which it has a field of view which extends across and slightly past the symbol. A portion of the reflected light which is reflected off the symbol is detected and converted into an electrical signal, and electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal from the photodetector may typically be converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
The decoding process in known scanning systems usually work in the following way. The decoder receives the pulse width modulated digital signal from the scanner, and an algorithm implemented in software attempts to decode the scan. If the start and stop characters and the characters between them in the scan were decoded successfully and completely, the decoding process terminates and an indicator of a successful read (such as a green light and/or an audible beep) is provided to the user. Otherwise, the decoder receives the next scan, performs another decode attempt on that scan, and so on, until a completely decoded scan is achieved or no more scans are available.
Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
Laser scanners are not the only type of optical instrument capable of reading bar code symbols. Another type of bar code reader is one which incorporates detectors based upon charge coupled device (CCD) technology. In such readers, the size of the detector is larger than or substantially the same as the symbol which is to be read. The entire symbol is flooded with light from the reader, and each CCD cell is sequentially read out to determine the presence of a bar or a space. Such readers are lightweight and easy to use, but require substantially direct contact or placement of the reader on the symbol to enable the symbol to properly read. Such physical contact of the reader with the symbol is a preferred mode of operation for some applications, or as a matter of personal preference by the user.
Among problems which are frequently encountered in the use of such laser beam devices which incorporate driving means, such as stepper motors, for "dithering" or essentially oscillating a scanning element or mirror in a reciprocatory or angular back and forth motion at high speeds, is the aspect that the scanning of the symbol is frequently distorted through vibratory effects or "jerky" motions which are superimposed on the scanning movement, and which are caused by the poles in the stepper motor, thus the movement of the scanning element or mirror, thereby tending to adversely influence the quality of the scanning process. Under ideal conditions, this angular reciprocatory or oscillating movement or "dithering" of the scanning element or mirror should be carried out to be as linear as possible during the operation of the scanning element. In essence, the speed of movement during the angular displacement should be as constant as possible over a reasonable scanning angle of less than 360.degree., preferably within the range of about .+-.5.degree. to .+-.7.degree. from a central or neutral position during the oscillation of the scanning element by the scanning motor.
Heretofore, attempts have been undertaken in the technology to overcome problems of this kind to some extent through the imposition of a D.C. current component on an alternating current which is applied to the coils of an electromagnetic stepper motor interacting with the permanent magnet thereof so as to cause the rotor of the stepper motor mounting the scanning element to "dither" or essentially oscillate in a somewhat more linearly oriented manner. In this connection, suitable types of wave forms for alternating currents may also be applied to the coils of the scanning motor in order to linearize the reciprocatory movement to the greatest possible extent; for instance, such as by utilizing a triangular A.C. wave form, amongst others.
However, although some degree of success has been achieved in linearizing the oscillating movement of such high speed scanning motors in order to reduce potential vibrations and adverse effects over scanning quality, the sudden reversal in direction at each end of the sweeping motion of the rotor shaft of the scanning motor mounting the scanning element or mirror, will prevent attaining any reasonable or satisfactory degree of linearity for such reversing oscillatory movement, and result in the generating of undesirable vibrations or jerky motions in the scanning element or mirror during operation of the scanning device.